Generally, an image processing apparatus for processing image data (for example, a personal computer or a workstation) can create various documents including characters, graphics, and images, with the use of application software. An image forming apparatus such as a printer, a fax machine, a copier, and a multifunction peripheral having functions of a printer/a fax machine/a copier, can form and output such documents as images by performing an image forming method such as an inkjet recording method or an electrophotographic method. In the image forming method, coloring material (image forming material) such as toner and ink is used to form (“record” and “print” may be used as synonyms) images.
When image forming apparatuses used in offices are used for outputting images, not only is the image quality of the printed images important, but printing costs are also important. In monochrome printing, the image forming apparatus basically uses a coloring material of only one color K (black). However, in color printing, the image forming apparatus uses coloring materials of four colors CMYK (or more than four colors when a special color is used). When outputting (expressing) a color image, the coloring materials may not always be used separately; plural coloring materials may be superposed in the same place to express various hues. Therefore, the cost required for color printing may amount to several times that of monochrome printing, according to a simple calculation.
Thus, an image forming apparatus usually has a printing mode (referred to as a “coloring-material usage amount reduction mode”; also referred to as an econo-mode, an ink-saving mode, a save mode, etc.) in which the amount of coloring materials used for printing out color images is reduced compared to the case of a regular mode. By having such a printing mode, costs for printing can be reduced.
For example, patent document 1 describes a process of setting a target for saving the usage amount of image forming materials based on specified conditions, and changing the color reproduction conditions with respect to the input color information based on the set target. For example, the color reproduction conditions are changed by changing the γ values of the color space, changing the color area compression conditions, or changing the directions of compressing the color areas.
Furthermore, patent document 2 describes a thinning-out process performed on printing data with the use of specific patterns. Patent document 3 describes a process of reducing gradations at a fixed rate with respect to input data. Patent document 4 describes a process of reducing the coloring-material usage amount to a predetermined amount, by reducing densities of objects (elements for forming images) in a descending order in terms of the coloring material consumption amounts of the objects (in a manner such that the outlines are darker than the inner areas). Patent document 5 describes a process of reducing the coloring-material usage amount when a coloring-material reduction mode is selected, by compressing gradation properties at a fixed rate. Patent document 6 describes a process of reducing the coloring-material usage amount when a coloring-material reduction mode is selected, by printing images in black to be as dark as those in a regular mode, and printing images in colors other than black to be lighter than those in a regular mode.
Patent Document 1: Japanese Laid-Open Patent Application No. 2006-68982
Patent Document 2: Japanese Laid-Open Patent Application No. 2006-270927
Patent Document 3: Japanese Laid-Open Patent Application No. H09-216419
Patent Document 4: Japanese Patent No. 3268712
Patent Document 5: Japanese Laid-Open Patent Application No. 2008-132665
Patent Document 6: Japanese Laid-Open Patent Application No. 2007-50708
However, in the above-described conventional technologies, the reduction amount of coloring material is not clear, and the image quality and the cost are not balanced. For example, when the coloring-material usage amount is reduced too much as a result of placing importance on cost, the entire image will have a light, blurry quality. Thus, the image will lack the intended sharpness of a color image. Conversely, if too much importance is placed on image quality, the amount of coloring material used cannot be sufficiently reduced.
As described above, in the conventional technology, the image quality may decline (due to less visible colors) as a result of reducing the coloring-material usage amount.